Substituent Effect of 4-Nitroimidazole Derivatives: Acidic Hydrogen as Modulator of the Nitro Radical Kinetic Stability

Abstract

The electrochemical reduction of 1-methyl-4-nitro-2-carboxyimidazole (RNO2COOH) in a nonaqueous medium produced two reduction signals, which were influenced by the dissociation of the compound. The first reduction signal was caused by the reduction of the acidic species, and the second signal was a quasi-reversible couple caused by the reduction of the conjugate base to the radical dianion. According to the theory developed by Nicholson for the study of the electrode reaction kinetics, we have obtained the heterogeneous rate constant, k(0)=(9.74 +/- 0.052)x10(-3) cm s(-1), for the reduction of the conjugate base to the radical dianion. The voltammetric behavior of RNO2COOH and its comparison with the behavior of another related compound such as 1-methyl-4-nitro-2-hydroxymethylimidazole is a good example to show the applicability of the general theory about the mechanism of self-protonation reactions in organic electrochemical processes to the specific case of nitroimidazole derivatives. According to the presented results, the presence of acidic hydrogen as a substituent in the nitroimidazole moiety would permit the modulation of the nitro radical anion stability, thus affecting its potential biological activity.